The infrared spectrum covers a range of wavelengths longer than the visible wavelengths but shorter than microwave wavelengths. Visible wavelengths are generally regarded as between 0.4 and 0.75 micrometers. Infrared wavelengths extend from 0.75 micrometers to 1 millimeter. The function of an infrared detector is to respond to energy of a wavelength within some particular portion of the infrared region.
Heated objects will dissipate thermal energy having characteristic wavelengths within the infrared spectrum. Different levels of thermal energy, corresponding to different sources of heat, are characterized by the emission of signals within different portions of the infrared frequency spectrum. Detectors for highest efficiency are selected in accordance with their sensitivity in the range corresponding to the particular detection function of interest to the designer. Similarly, electronic circuitry that receives and processes the signals from the infrared detector must also be selected in view of the intended detection function.
A variety of different types of infrared detector have been proposed in the art since the first crude infrared detector was constructed in the early 1800's. Virtually all contemporary infrared detectors are solid state devices constructed of materials that respond to infrared energy in one of several ways. Electro-optical detectors, for example, respond to infrared energy by absorbing that energy, which causes electrical charge carriers to be generated. These carriers, in turn, can be detected by a change in the electrical properties of the material, such as a change in resistance. By measuring this change the infrared radiation can be derived. Advances in semiconductor materials and the development of highly sensitive electronic circuitry have advanced the performance of contemporary infrared detectors close to the ideal photon limit.
Current infrared detection systems incorporate arrays of large numbers of discrete, highly sensitive detector elements, the outputs of which are connected to sophisticated processing circuitry. By analyzing the pattern and sequence of detector element excitation, the processing circuitry can identify and monitor sources of infrared radiation.
Though the theoretical performance of such systems is satisfactory for many applications, it is difficult to actually construct structures that mate a million or more detector elements and associated circuitry in a reliable and practical manner. Consequently, practical applications for contemporary infrared detection systems have necessitated that further advances be made in areas such as miniaturization of the detector array and accompanying circuitry, minimization of noise intermixed with the electrical signal generated by the detector elements, and improvements in the reliability and economical production of the detector array and accompanying circuitry.
A contemporary subarray of detectors may, for example, contain 256 detectors on a side, or a total of 65,536 detectors, the size of each square detector being approximately 0.0035 inches on the side with 0.0005 inches spacing between detectors. Such a subarray would therefore be 1.024 inches on a side. Thus, interconnection of such a subarray to processing circuitry requires a connective module with sufficient circuitry to connect each of the 65,536 detectors to processing circuitry within a square, a little more than one inch on a side. The subarray may, in turn, be joined to form an on-focal plane array that connects to 25 million detectors or more. Considerable difficulties are presented in aligning the detector elements with conductors on the connecting module and in isolating adjacent conductors in such a dense environment.
The outputs of the detectors must undergo a series of electronic processing steps in order to permit derivation of the desired information. The more fundamental processing steps include preamplification, tuned bandpass filtering, clutter and background rejection, multiplexing and fixed noise pattern suppression. By providing a signal processing module that performs at least a portion of the processing functions within the module, i.e., on integrated circuit chips disposed adjacent the detector focal plane, the signal from each detector needs be transmitted only a short distance before processing. As a consequence of such on focal plane or up front signal processing, reductions in size, power and cost of the main processor may be achieved. Moreover, up front signal processing helps alleviate performance, reliability and economic problems associated with the construction of millions of closely spaced conductors connecting each detector element to the main signal processing network.
Infrared detectors are typically fabricated from single crystalline semiconductor wafers by photolithographic techniques and then bump bonded to signal processing modules. Each signal processing module is a stack of thin substrates which contain signal processing circuitry. Contact pads on one edge of a substrate are matched to a row of infrared detectors. A plurality of such modules may be stacked to form a subarray. A plurality of subarrays may then be assembled to form an infrared detector focal plane array.
A common problem encountered in stacking the substrates to form a module is in the alignment of the substrates. The spacing of the substrates in the stack must match the spacing between rows of detector elements.
Alumina ceramic is used extensively as the substrate. A glass adhesive between adjacent ceramic substrates in the module stack is preferred to maintain the substrate spacing for two reasons:
(1) In the case of an infrared imager that is operated at a cryogenic temperature it is highly desirable to have the coefficient of thermal expansion of the laminate adhesive close to that of the substrate to maintain the substrate spacing A glass adhesive possesses a thermal coefficient close to alumina ceramic.
(2) A manufacturing process using a glass adhesive may consist of first screen printing a layer of glass paste onto the ceramic. After the paste solvent and other organics are evaporated and burned out, the glass is glazed by firing. This glass layer is ground to the required glass-plus-ceramic thickness, which matches the spacing between rows of detectors.
Glass coated substrates are conventionally stacked and then fired under pressure to bond the substrates together. This bonding, second firing process, however, can result in marginal adhesion from poor contact between the glass and the ceramic caused by voids, gaps or insufficient wetting. Raising the firing temperature to increase the flow of the glass or increasing the pressure are means of reducing these factors. However, these means can cause the glass to flow out of the stacked layers and so to reduce the glass layer thickness and to lose the required precise substrate spacing. Furthermore, gaps and voids in the glass layer present a difficulty in making patterned bonding pads on the face of the module where connection is made to the detector array.
Thus it would be desirable to provide a method to obtain the substrate spacing in the module stack that precisely matches :he detector element spacing and simultaneously to obtain strong substrate adhesion.